1. Field of the Invention
The present invention relates to a distance measuring apparatus. More specifically, the present invention is preferably applied to a distance measuring apparatus provided in an apparatus which is required to have high precision in positioning; for example, a sample stage of a projection exposure apparatus.
2. Related Background Art
Conventionally, in a projection exposure apparatus for manufacturing semiconductor circuits or liquid crystal devices, a plurality of exposed regions (circuit pattern formation regions) which are arranged in a matrix on a photosensitive substrate (a semi-conductor wafer or a glass plate on which a resist layer is formed) are exposed in sequence through patterns of a plurality of corresponding masks (reticles) to light by means of a projection optical system in order to transfer and overlap said patterns onto the exposed regions.
Positioning (i.e. alignment) of the projected images of said plurality of reticle patterns is usually performed by irradiating marks for alignment formed on the reticles and marks for alignment formed on the substrate with illuminating light and determining the amount of relative divergence therebetween. In this case, in order to detect the position of a movable stage (plate stage) on which the substrate is mounted, length measuring devices of laser beam interference type (hereinafter referred as interferometers) which utilize laser beams are employed (FIG. 8).
More specifically, a projection exposure apparatus 1 is provided with interferometers 3 and 4 which are arranged in a plane parallel with a mount surface 2A of an XY-stage 2 on which a substrate is mounted, wherein optical axes of the interferometers are orthogonal so that the interferometers 3 and 4 measure the shifting amounts in the X-axis direction and the Y-axis direction, respectively.
The interferometer 3 comprises a fixed mirror 5 fixed to an immobile part of the XY-stage 2, a movable mirror 7 fixed to a movable part (of the XY-stage 2) and a beamsplitter 9. Similarly, the interferometer 4 comprises a fixed mirror 6, a movable mirror 8 and a beamsplitter 10. The shifting amount in the X-axis direction is constantly measured on the basis of interference between light reflected by the fixed mirror 5 and light reflected by the movable mirror 7. Similarly, the shifting amount in Y-axis direction is constantly measured on the basis of interference between light reflected by the fixed mirror 6 and light reflected by the movable mirror 8.
The movable mirrors 7 and 8 have to be constantly illuminated with laser beams to measure the shifting amounts of the XY-stage 2 in the X-axis direction and the Y-axis direction by means of the interferometers 3 and 4.
Therefore, the area within which the XY-stage can move should be narrower than the lengths of the movable mirrors 7 and 8.
More specifically, the movable range in the X-axis direction is limited to the length lx of the movable mirror 7 or less, and the movable range in the Y-axis direction is limited to the length ly of the movable mirror 8 or less.
Accordingly, the lengths of the movable mirrors 7 and 8 should be made greater to enlarge the area in which the XY-stage can move. It is difficult, however, to work a surface of a large movable mirror with high precision or to manufacture such a large movable mirror. So, an XY-stage 2 of this kind of apparatus which can move within a large movable range has not been manufactured.